The present invention relates to a polyamide composition, which is excellent in thermal stability and is suitable for industrial materials such as various mechanical parts and electric/electronic parts, and a process for producing the same.
When a polyamide resin undergoes various heat histories, thermal degradation and oxidative degradation occur to result in increase of yellowness, change in molecular weight, and decrease in mechanical properties such as toughness and durability. The various heat histories occur because of polymerization, melt-kneading, molding (injection, extrusion, blowing, fiber-spinning, film formation, etc.), use in a high temperature environment, and the like. For the purpose of reducing the degree of degradation during the heat histories, a method of blending a phosphorus compound acting as a heat stabilizer with a polyamide composition is well known and frequently used. On the other hand, it is also well known that the phosphorus compound acts as a polymerization catalyst for polyamide resins. Therefore, a method of increasing the molecular weight and inhibiting increase in yellowness by blending a phosphorus compound such as sodium hypophosphite in a polymerization process for polyamide resins is a well-known technology in the art. However, a polyamide resin blended with only a phosphorus compound not only exhibits insufficient effect of inhibiting increase in yellowness but also induces a large change in the molecular weight at every heat history, particularly melt-kneading or a heat melt step during molding. These problems occur because the catalytic action of the phosphorus compound still remains. Yellowing and/or large changes in molecular weight cause problems such as decreased productivity, defective products, and the like.
As methods for solving these problems, methods of using a phosphorus compound and another metal compound in combination are disclosed in Japanese Patent No. 2741795 and JP 9-512839 T. More specifically, Japanese Patent No. 2741795 discloses a production process wherein a phosphorus compound such as sodium hypophosphite and a Group 1 base such as sodium bicarbonate are blended with a polyamide-forming reactant (starting material) and then polymerization is conducted or they are added to a polyamide in a melted state. The Group 1 base in the publication is selected from hydroxides, oxides, carbonates, alkoxides, bicarbonates, and hydrides. Moreover, JP 9-512839 T discloses a method wherein a phosphorus compound such as sodium hypophosphite and a polyvalent metal compound such as calcium acetate are blended with a polyamide-forming reactant (starting material) and then polymerization is conducted or they are added to a polyamide in a melted state. The polyvalent metal compound in the publication is selected from halides, nitrate salts, and carboxylate salts (i.e., acetate salts, propionate salts, benzoate salts, stearate salts, and the like) of Group 2 metals, zinc, and aluminum. It is disclosed that the polyamide resins obtained by these two production methods inhibit increase in yellowness even after storage for a long period of time and repeated melting operations during molding and extrusion as compared with conventional polyamide resins. Moreover, it is also described that change in molecular weight during the repeated melting operations is inhibited based on decrease in solid-phase polymerization rate. However, from the investigation by the present inventors, it has been found that, even when these methods are employed, if melting operations are repeated, then not only the effect of inhibiting increase in yellowness and the effect of inhibiting change in molecular weight are insufficient but also mechanical properties, such as toughness, is not well satisfactory.
On the other hand, JP 47-39156 A, JP 49-116151 A, and JP 1-104652 A disclose polyamide resins with which a soluble aluminate such as sodium aluminate is blended. Even when only a metal aluminate is blended with a polyamide-forming component (starting material) and then polymerization is conducted according to the technology, a prolonged polymerization time is required for obtaining a desired molecular weight owing to decrease in polymerization rate and, as a result, increase in yellowness and decrease in mechanical properties such as toughness are induced. Moreover, even when a metal aluminate is blended with a polyamide by melt-kneading, the molecular weight of the resulting polyamide resin tends to decrease and, as a result, the polyamide has insufficient mechanical properties such as toughness.
The present invention relates to a polyamide composition excellent in thermal stability, which is suitable for industrial materials such as various mechanical parts and electric/electronic parts, and a process for producing the same. More precisely, it relates to a polyamide composition which, even after a prolonged heat history or repeated heat histories, is inhibited from increasing in yellowness, is inhibited from suffering thermal decomposition, has a stable melt viscosity, and is excellent in mechanical properties such as toughness, as compared with conventional polyamide resins.